Fire and Landslides

WTypes of Wildfires

1. Crown Fire: the tops, or the canopy of trees are burned.
2. Surface Fire: organic matter on the ground, such as leaves and grass are burned.
3. Ground Fire: everything is burned.

Surface Fire

• Burns plants, shrubs, trees and all decaying matter on the ground
• Surface fires are the “firestorm” events seen during large scale wildfires.

Effects on bedrock

• Rock weathering (spalling)
  • Depending on the rock type, flakes of up to 3 cm become detached from the rock after being heated to extreme temperatures.
  • Sandstones and limestones are weathered more severely than metamorphic and intrusive igneous rock.
• Vertical Fracturing
  • The rock will fracture depending on rock type, size, and sustained heat. This happens in the later stages of the fire,and after cooling has started.
  • Rock can fracture days or months after the fire has finished.

Dry Ravel

• Dry material moving downhill by gravity, usually on slopes with an angle >25°
• With vegetation, the dry ravel is stored higher on the slope of the mountain or hill, but after a fire, there is nothing to slow down the material.

Vegetation is removed

• All vegetation is removed in severely burned areas.
• All “litter” is burned. Litter is the layer of dead and dying vegetation found on the surface of the soil.

Hydrological changes

• No vegetation =
  • No Transpiraton
  • No Interception
  • Less surface storage of water
  • Increased invertebrate activity
  • Creation of Hydrophobic Soil Layer
• Transpiration
  • The removal of water from the soil by plants.
  • Plant roots absorb water in the soil
  • With the plant roots dead and decaying, no water is removed from the soil in this fashion, nor does the roots bind the soil as would live plants.
• Interception
  • The holding of raindrops by plants as the water falls onto leaves, stems, and branches. When the plant can hold no more, the water will drip from the plant or run down the stem.
  • With no vegetation, the rain fall hits the bare earth.
• Surface storage of water
  • Litter stores water: all of the dead leaves on the ground, and other organic materials, absorb water.
  • The fire has burnt the litter, therefore, no water is absorbed in that fashion, water his bare earth during rainfall.
  • The vegetation is gone, thus, no transpiration, no interception, therefore, the only substance left to absorb rainwater and runoff is the burnt remains and the soils.
  • This leads to less surface storage of water.
• Increased invertebrate activity
  • During the fire, all of the bugs try and burrow down into cooler soils. Most get burned up, however, some are relatively safe in the cooler soil.
  • The burrowing leaves tunnels in the soil, which make the soil more subject to erosion.
• Hydrophobic layer
  • High heat causes hydrological change to the soil
  • Different soil types react differently at varying temperatures
  • Therefore, the content of hydrophobic soil varies from areas at risk to wildfire.
  • The hydrophobic layer is a layer of soil that is water repellant, not permeable.
  • The soil is formed during the wildfire.
  • At high temperatures experienced in a fire, the resins, oils, and fats stored in vegetation and litter are vaporized. These vapors are released to the atmosphere, as well as into the soil. The vapors in the soil condense into the lower, cooler, soil under the ground and reform into the hydrophobic layer, a water repellant layer.

Recipe for movement

• Dry ravel
• No vegetation
• No water storage capacity
• Hydrophobic layer
• Spalling, burnt debris, loose materials
• Steep slope
• High magnitude – low frequency storm

Rainsplash

• The impact of raindrops on the soil may break down soil peds, loosen particles, and cause turbulence in the sheet wash of water flowing down slope.
• Sheet wash, or sheet erosion, is a thin film of water transporting soil particles by rolling them down hill.

Landslide

• Rain or snowmelt is added to the burnt hilside
• The water saturates the top layer of soil
• The slope has a hydrophobic layer
• There is no vegetation to hold the soil in place
• The soil then slumps, or slides, downhill
• Since it takes time for vegetation to re-establish on steep slopes, the risk of landslides prevails or up to 20 years in dry climates
• Landslides can also be re-activated over time with further erosion of channels, slopes being further eroded, which can cause the toe of the slide to be removed, triggering another landslide.

Debris flow

• The very rapid down slope movement of debris with high water content, with a course guided by stream channels.
• Debris flows tend to occur with little warning, and can be extremely destructive.
• Process
  • Severe Wildfire occurs
  • Dry Ravel produces progressive accumulation of sediment
  • Rain storm event
  • Rainsplash occurs directly with bare earth
  • Small rills are cut into the hill side.
    • Small channels between 5 – 2000 MM in width, and very closely spaced, they develop well in areas of heavy rainfall. Rills can form into gullies, and eventually into mini-rivers.
  • The rills form into gullies of water and small sediment debris
  • More sediment is picked up and channeled through the gully, which continues to cut down into the hillside.
  • Water and debris pick up velocity down hill, and continue to cut a channel, grabbing larger and larger debris, boulders.

Areas dealing with Post-fire hazards

• Big Sur
  • 1⁄4 million acres burned this summer
  • Very Steep slope hillsides
  • Setting up catch basins for rocks and debris
  • Debris racks to catch boulders and trees and shrubs
  • Preparing for winter rains, weather alert system for downstream residents.
• Santa Barbara County
  • Quoted from LA Times, 09-12-08
  • “Goleta…soon will be coated aerially with a glue-like substance that is embedded with fertilizer. The mixture of wood and paper fibers, water and a plant-based binder will absorb rainwater and coax faster regrowth of native chaparral.”
• Butte County
• So Cal
• Tahoe
• Many many many many more

Post-fire Safety Efforts

• The US Forest Service and the USGS work diligently to analyze post-wildfire hillslope and stream channel conditions. They will determine is an emergency has been created after the wildfire has burned. Satellite imagery and GIS Software are used as well.
• A few aspects analyzed include:
  • Soil Properties
  • Burn severity
  • Hydrologic run-off
  • Slope
  • Vegetation layers
  • Unique hydrologic response
  • Soil erosion characteristics